Solution:

a) The Biological Oxygen Demand (BOD) will increase, resulting in the mortality of species such as fish due to a lack of oxygen in the water. As people drink water directly from the river, diseases like cholera and dysentery will spread.

b) Anaerobic sludge digestion is important in sewage treatment because it reduces Biological Oxygen Demand (BOD). Sludge digestion occurs in the presence of anaerobic bacteria, and when the sludge is digested, the anaerobic bacteria release a mixture of gases such as methane (), carbon dioxide (), and hydrogen sulfide (). Biogas is made up primarily of these components.